Wyckoff — Crystal Structure of Ammonium Chloride. 473 



Conclusions. — The conclusions which were given previ- 

 ously can now be stated in even more definite terms. 8 

 If the hydrogen atoms of ammonium chloride have an 

 arrangement which conforms to the symmetry of the 

 crystal as a whole, then this symmetry must be hemi- 

 morphic hemihedral (tetrahedral) ; if, as is perhaps con- 

 ceivable, the hydrogen atoms have a more or less hap- 



Fig. 3. 



©CL #N #H 



Figure 3. The unit cube of ammonium chloride. 



hazard distribution about the nitrogen atoms, its 

 symmetry will be holohedral. In no case, however, can 

 the symmetry of the arrangement of the atoms of this 

 crystal agree with the enantiomorphic hemihedry which 

 studies of both face development and etch figures assign 

 to it. If these crystallographic data are correct, it will 

 therefore be necessary to conclude that such studies even 

 when carried out under ideal conditions to yield the 

 maximum amount of data conceivable can not always 

 indicate the symmetry of the arrangement of the atoms 

 within a crystal. 



This conclusion is not only of significance to crystallog- 



8 Ralph W. G. Wyckoff, this Journal, 3, 177, 1922. 



Am. Jour. Sci.— Fifth Series, Vol. IV, No. 24.— December, 1922. 

 31 



